Spermatogenesis is a highly organized and productive self-renewing system that produces millions of sperm each day throughout postpubertal life. Age-related declines in spermatogenesis are manifested as decreases in sperm number and quality and loss of fertility. Spermatogonial stem cells (SSCs) are at the foundation of the spermatogenic lineage, and we hypothesize that aging of the SSC pool causes age-related reproductive deficits. A spermatogonial stem cell transplantation technique was developed nearly a decade ago and allows investigators to remove stem cells from their cognate young or aged niched and transplant them to a young or old recipient environment. In this way it is possible to distinguish intrinsic stern cell characteristics and extrinsic environmental effects. The SSC functional assay will be used to enable four specific aims designed to identify functional correlates between stem cell activity and histological and molecular hallmarks of aging in the mouse testis. (1) Evaluate spermatogenesis in the testes of young and aging 129 x C57 mice, histologically, immunochemically and functionally (by breeding). Characterize spermatogonial stem cell activity in the corresponding young and aged donor testes using SSC transplantation, (2) Characterize the quantity and quality of stem cell niches in young and aging recipient testes. Evaluate whether transplanted young stem cells can restore spermatogenesis in aged infertile testis environments, (3) Assess the immortality of stern cells by serial retransplantation of spermatogonial stem cells into perpetually young testis environments, (4) Examine molecular hallmarks of aging in the stem cells of young and aging testes. It is anticipated that the results of the proposed studies in the mouse testis model system will provide general insight into the role of stem cells in the aging process. Furthermore, the new knowledge may provide leads for the development of stem cell-based therapies for age-related health concerns.